Methods for the production of ortho-dialkylated phenols such as 2,6-diisopropylphenol are well-known and reported in the literature. The most efficacious process involves reacting phenol with an olefin such as propylene using an aluminum phenoxide catalyst, such as described in U.S. Pat. No. 2,831,898 to G. C. Ecke and A. J. Kolka. Modifications of this original phenol ortho-alkylation chemistry subsequently appeared, primarily involving catalyst alterations or modifications. In all such processes it is possible to achieve good selectivity in the production of the 2,6-dialkylphenol. Nevertheless the crude product mixtures typically contain impurities such as unreacted phenol, one or more monoalkylphenols, one or more dialkylphenol isomers other than the desired 2,6-dialkylphenol isomer, 2,4,6-trialkylphenol and phenolic ethers.
Recently, substantial commercial requirements for highly pure 2,6-diisopropylphenol (99.8% or more) have arisen. To fulfill these requirements on an economical basis, a concomitant need has arisen for technology enabling economical, large scale purification of crude phenol ortho-alkylation reaction product mixtures.
Production of high purity (99.8% and above) 2,6-diisopropylphenol by distillation is an extremely difficult operation--much more difficult than might first appear. A complicating factor is that other impurities are readily generated by oxidation reactions which occur at the elevated temperatures required during the distillation. These impurities include 2,6-diisopropylbenzoquinone, 2-isopropyl-6-isopropenylphenol, 2,6-diisopropenylphenol and 2,2-dimethyl-4-isopropyl-1,3-benzodioxole.
Laboratory studies have indicated that the formation of these impurities can be prevented if oxygen is totally excluded from the system. This, however, is not practical in most industrial distillation facilities carried out under vacuum, due to seepage of air through standard pipe flanges and fittings.
The formation of these and other impurities can be minimized by use of continuous rather than batch distillation, because the residence time--i.e., the time the material is held at the elevated temperatures--is much less in a continuous distillation. However in a situation of this kind, traditional continuous distillation alone requires two distillation columns to accomplish this separation, whereas batch distillation requires only a single column. Because of the considerable capital investment required for distillation columns, a batch distillation would be much preferred were it not for the longer durations of exposure of the material to high temperatures and the practical difficulty of rigorously excluding air in such operations when conducted on a large scale.
U.S. Pat. No. 5,175,376 to K. M. Niemenen and P. K. Essen describes a purification procedure for 2,6-diisopropylphenol which involves subjecting the impure 2,6-diisopropylphenol to crystallization at a temperature in the range of about -25.degree. to about 18.degree. C. at which 2,6-diisopropylphenol crystallizes and the impurities do not, filtering and washing the 2,6-diisopropylphenol preferably with a non-polar aliphatic hydrocarbon. The solvent is removed from the product by distillation, and the product itself is recovered as a single fraction in the distillation. Such a procedure is not well suited for use in a large scale commercial operation.
U.S. Pat. No. 5,264,085 to M. Inaba, Y. Higaki, K. Jinno, M. Kataoka N. Sato and M. Honda describes a method of continuously separating components of a hydrous phenols mixture containing methanol by distillation. The method involves recovering methanol from the top of a single distillation column, dragging water containing phenols as a side stream from the recovery section of the distillation column and the dehydrated phenols as a bottom product.